The concept of a closed fuel cycle in nuclear power stations provides disposal of all radioactive waste while obtaining products suitable for long-term storage or burial, for example, radionuclides in a bonded condition in hardened materials or suitable for using their radioactivity in future, for example, as isotope products.
The basic mass of radionuclides after regeneration of irradiated nuclear fuel is in the form of high-level liquid waste, which, alongside with a high contents of stable macro admixtures—sodium, aluminum, iron, rare-earth and other elements in an amount of up to 300 g/l, contain a small amount of radionuclides of cesium, strontium and transuranium elements.
To reduce the volumes of hardened materials to be buried but containing radionuclides with a different degree of activity, for example, presently used borosilicate or phosphatic glasses, it is necessary to increase the specific activity of radionuclides in the glass volume. The specific activity of solutions used for solidification and subsequent burial, can be increased by reducing the amount of salts in the solutions of radionuclides or by adding saltless fractions of strontium and cesium radionuclides to the solutions of radionuclides to be buried.
At the present time, the most advanced method of fractionating liquid radioactive waste based on extraction of radionuclides including strontium and cesium radionuclides can be realized in two ways.
One of them is based on application of polyhedral carborane complex, in which cobalt dicarbolide chloride (DCC) is used as complexing agent for extraction of cesium, while strontium is extracted using high-molecular polyethylene glycols dissolved in fluorinated nitroaromatic solvent. However, this technology has significant disadvantages involved in a necessity of preliminary adjustment of a starting solution, high losses of extractants in the process of isolation of refined products and isolation of the target products, and a high cost of extractant and latent solvent.
Another way is associated with a possibility of using complexing agents consisting of macrocyclic polyethers or crown ethers of a different structure having unique selectivity to radionuclides including that to ions of strontium-90 and cesium-137 with a wide range of applicable solvents of crown ethers for creation of extraction mixtures.
However, practical application of crown ethers is rather limited due to low solubility of crown ethers themselves, particularly those containing benzene substituents in organic solvents and high solubility of crown ethers and their complexes in aqueous solutions.
In order to increase the solubility of crown ethers in organic solvents and to decrease their solubility in aqueous solutions, macrocyclic compounds were synthesized from a group of crown ethers having aromatic fragments, containing alkyl and/or hydroxyalkyl substituents of a linear and/or branched structure, and/or cyclohexane fragments containing alkyl and/or hydroxyalkyl substituents of a linear and/or branched structure, and/or fragments of —O—CHR—CH2O—, where R is the normal or branched alkyl or hydroxyalkyl.
The most wide-spread structures include: bis(tert-butylbenzo)-18-crown-6 (DTBDB18C6), di-isooctylbenzo-18-crown-6 (DIODB18C6), dibenzo-21-crown-7 (DB21C7), bis-4,4′(5′)[1-hydroxy-2-ethylhexyl]benzo-18-crown-6 (CROWN XVII), bis(tert-butylcyclohexane-18-crown-6 (DTBDCH18C6), di-isooxyldicyclohexane-18-crown-6 (DIODCH18C6), bicyclohexane-18-crown-6 (DCH18C6), 6c-4,4′(5′)[1-hydroxyheptyl]cyclohexo-18-crown-6 (CROWN XVI), dibenzo-18-crown-6 (DB18C6), dibenzo-24-crown-8 (DB24C8), dibenzo-30-crown-10 (DB30C10). Except for DB18C6, DB21C7, DB24C8 and DB30C10, all listed crown ethers have higher solubility in organic, especially, paraffin, solvents and smaller losses at water-organic contacts preserving, according to the published data, high selectivity to cations of radionuclides being extracted.
Known in the art are methods of extraction of cesium from aqueous solutions containing other ions by means of extraction mixtures based on crown ethers bis-4,4′(5′)-(hydroxyalkyl-benzo)-18-crown-6 in an organic solvent containing tri-n-butyl phosphate or methyl isobutyl ketone with subsequent contact of the obtained organic solution with a solution of an inorganic acid (U.S. Pat. No. 5,888,398, A).
Known in the art are methods of extraction of cesium from aqueous alkaline waste also containing a considerable amount of other ions of alkali metals such as sodium and potassium using extraction mixtures based on calixarene crown ethers in a neutral hydrocarbon solvent, for example, crown ether calix [4]arene-(bis-tert-octylbenzo-crown-6) ether in aliphatic kerosene (U.S. Pat. No. 6,174,503, B1).
Known in the art is a method of selective extraction of cesium-135 by an extraction mixture containing a crown-ether tert-butyl benzo-21-crown-7 (TBB21C7) and sodium tetraphenyl borate (FR, 2700709, A1).
Also known in the art is an extraction mixture for extraction of cesium from aqueous nitrate mediums containing a crown ether dibenzo-21-crown-7 in a concentration of 0.1-0.2 mole/l in an organic dissolvent 2,2-dihydrotrifluoroethylene pentafluoroethyl ether (SU, 1693438, A1).
Known in the art are methods of consecutive extraction of cesium and strontium from acidic solutions of a radioactive waste: cesium extrantant, consisting mainly of crown ethers bis-4,4′(5′)[1-hydroxy-2-ethylhexyl]benzo-18-crown-6 (subsequently referred to as Crown XVII) with liquid cation-exchange dinonylnaphthalene sulfuric acid or didodecylnaphthalene didodecylnaphthalene sulfonic acid, and extraction of strontium by an extrantant consisting mainly of a crown ether bis-4,4′(5′)[1-hydroxyheptyl]cyclohexo-18-crown-6 (subsequently referred to as Crown XVI) with the same cation exchangers in a matrix solution containing tributyl phosphate and kerosene (U.S. Pat. No. 4,749,518, A). A disadvantage of this method of joint extraction of strontium and cesium is a low distribution ratio of strontium and cesium in the process of extraction and isolation of radionuclides because of a low radiation resistance of the extraction mixture components.
Known in the art is a method of isolation of strontium, neptunium, americium and plutonium from aqueous nitrate solutions by an extraction mixture containing a crown ether, for example, 4,4′(5′)[R, R′]bicyclohexane-18-crown-6, where R and R′ are selected from the group including methyl, propyl, isobutyl, t-butyl, hexyl, heptyl, and n-octyl(phenyl)-N,N-diisobutylcarbamoyl phosphine oxide in a solvent selected from the group containing normal paraffin hydrocarbons and isoparaffin hydrocarbons (U.S. Pat. No. 5,169,609, A).
However a common disadvantage of the above-described methods is a low radiation resistance of the extractants in a continuous extraction cycle.
Also known in the art are methods of extraction of radionuclides including strontium-90, cesium-135 and cesium-137 with the help of extractants containing crown ethers with various solvents and resistant to radiation.
For example, a method is known, which is used for extraction of metals, such as plutonium, uranium and strontium, from aqueous solutions using cis-sin-cis isomer of crown ether bicyclohexane-18-crown-6 (DCH18C6) in an organic solvent such as nitrobenzene or ethylene dichloride, or in a solid phase such as silicon (FR, 2656149, A).
Known in the art is a method of neutralization of radioactive nitrate solutions containing strontium and sodium in a concentration of 0.01 to 0.2 mole/l with the help of a mixture containing crown ether bicyclohexane-18-crown-6 (DCH18C6) and a solvent, such as chloroform CHCl3, 1,1,2,2-tetrachloroethane, dichloromethane, nitrobenzene, brombenzene placed in cation-exchange resin (FR, 2707416, A1).
However, the application of said extraction mixtures and solvents, though has shown high radiation resistance of the extractants and distribution ratios of strontium and cesium acceptable for industrial use, still has not eliminated considerable losses of crown ethers due to their washing off in the process of re-extraction of radionuclides by aqueous solutions.